Aqueous bath and method for deposition of copper by chemical reduction



United States Patent 3,454,416 AQUEOUS BATH AND METHOD FOR DEPOSITION OF COPPER BY CHEMICAL REDUCTION Kurt Heymann, Giinter Woldt, and Helga Griindel, Berlin, Germany, assignors to Schering A.G., Berlin, Germany No Drawing. Filed Dec. 22, 1965, Ser. No. 515,753 Claims priority, application Germany, Feb. 20, 1965, Sch 36,568 Int. Cl. B4411 1/34; C23c 1/10 US. Cl. 11747 5 Claims ABSTRACT OF THE DISCLOSURE wherein X is S or NH; R is hydrogen, lower alkyl, phenyl, carboxy lower alkyl, or amino; and R is hydropen, lower alkyl, or nitro. Also disclosed is a method for depositing copper on a receptive base from this solution.

This invention relates to the deposition of copper by chemical reduction. More particularly, this invention relates to metallizing, namely to the deposition of copper from aqueous solutions of its salts by chemical reduction.

Copper is deposited by chemical reduction on such nonconductors as plastics or ceramics to provide a conductive primer coating on which heavier metallic deposits may then be formed by electrodeposition. The solutions employed, generally contain an inorganic copper salt as a source of copper, a complexing agent, normally the salt of an organic acid or an organic base to form complex ions with copper, and a reducing agent.

The known copper deposition solutions heretofore used are relatively unstable and decompose within one or two days. Their copper content is precipitated spontaneously, either as metallic copper or as a copper oxide. Stabilizers have been proposed heretofore, but have been found to be of limited value. The known stabilizers only moderately extend the useful life of the copper depositing solutions, and the relatively more effective known stabilizers tend to cause discoloration and other deterioration of the copper deposits formed. Yet, even the best known stabilizers do not permit uniform conditions to be maintained for continuous operation of the copper deposition process.

It has now been found that the known solutions for copper deposition by chemical reduction can be stabilized to a degree heretofore unavailable by the addition of small amounst of compounds of the formula:

wherein X is S or NH; R is hydrogen, lower alkyl, phenyl, carboxy-lower alkyl, or amino; and R is hydrogen, lower alkyl, or nitro.

3,454,416 Patented July 8, 1969 The conventional copper deposition solutions when containing small amounts of the above compounds are stable enough to permit continuous metallizing with appropriate replenishment of the copper salt lost by deposition, and of other constituents of the bath, such as complexing and stabilizing agents, lost by drag-out. These losses are readily predictable for established processing conditions, and can be balanced by the periodic additions of suitable concentrates of copper salt, normally copper sulfate, and complexing agent, and of stabilizing agent.

The copper deposition solutions of the invention are stable not only at room temperature, but also even at elevated temperatures which are known to be deleterious to the conventional copper deposition solutions, thus permitting operation at temperatures at which the deposition rate is faster than at room temperature. The copper coatings formed from the solutions of the invention are bright red and consist of pure metal.

The stabilizing agents of the invention are known chemical compounds, and are prepared by known methods, rhodanine, for example, is readily obtained by reacting salts of dithiocarbamic acid with a halocetic acid in aqueout solution and subsequent heating in the presence of hydrochloric acid. Thiohydantoin is formed at elevated temperature from glycine and isothiocyanates.

The following compounds are typical stabilizing agents for copper deposition solutions:

Rhodanine N-methylrhodanine N-phenylrhodanine Rhodanine-N-acetic acid N-aminorhodanine 3-phenyl-5-nitrorhodanine S-ethylrhodanine Thiohydantoin Rhodanine and N-methylrhodanine have been found to stabilize copper deposition solutions over the longest periods, and are capable of preventing decomposition of such solutions over practically unlimited periods.

The stabilizing agents of the invention may be employed in concentration between one milligram and 0.5 gram per liter, but they are so effective in small amounts that nothing is gained by using more than about 0.05 gram per liter. The concentration of the conventional constituents of the solutions may be varied within the usual limits without affecting thestabilizing effect of the aforedefined compounds, and the specific nature and concentration of the copper salts, complexing agents, reducing agents, buffers, wetting and brightening agents employed is not critical.

The presence of the stabilizing agents does not aifect the mode of operation of the deposition solution, and the deposits formed have the expected electrical conductivity so that heavier metal deposits can be formed thereon by electro-deposition. Metals as well as non-conductors may be metallized in copper deposition solutions of the invention.

The coatings produced by chemical reduction in the presence of the aforedescribed stabilizing agents are distinguished by their clear, bright red, metallic appearance from the relatively dark and grainy deposits containing significant amounts of copper oxides which are characteristic of solutions containing the known 2-mercaptobenzthiazole as a stabilizing agent.

The following table lists the compositions of five typical copper deposition solutions of the invention.

TABLE Grams per liter Examples I III 011804. 5Hz0 Sodium-potassium tartrate Disodium ethylenediamine- N-methylrhodanine. N-aminorhodanine The stabilized copper deposition baths of the invention do not decompose spontaneously when stored over extended periods. The solutions whose compositions are given in Examples I and II above did not show measurable evidence of decomposition when stored for more than three months.

It will be appreciated that such stability is possible only in the absence of metals which promote deposition of copper. Suitable tan-ks for storage are therefore such which consists of non-metallic vinyl chloride, polyethylene, polypropylene, or similar plastics which are nonconductors and are resistant to chemical attack by the copper solutions of the invention.

When brought into contact with metals or non-metals suitably prepared in a known manner, the solutions of the invention promptly cause the deposition of copper coatings.

The solutions are preferably used having a specific gravity of about 1.04, a pH value of 12.0 to 12.5, and at a temperature of 20 40 C. The containers employed are either plastic tanks or steel tanks coated with suitable plastics. Mechanical agitation is preferred to obtain or effect greater uniformity of the copper coatings. Furthermore, air agitation is not advisable so as to avoid oxidation. The usual time of immersion for steel objects is preferably about to 20 minutes, and a deposition time of more than 20 minutes is not usually needed.

The deposition rate in the typical solutions of Examples I and II is 0.3 micron in 10 minutes at 20 C., and 0.6 micron in 20 minutes. At 40 C., the deposition rate rises to 0.8 micron in 10 minutes, and 1.3 microns in 20 minutes.

It is to be noted that the rate of deposition decreases with the use of the solution, and can be increased by raising the temperature if the time needed for adequate coverage becomes excessive. The solution composition is best controlled by determining the copper content by analysis, and making additions of a concentrated stock solution to restore the original copper content.

The copper coatings produced by immersion in the solutions of the invention may be covered by electrolytically produced metal layers in a conventional manner.

The methods of preparing non-conductors for copper deposition in the solutions of the invention are known in themselves. Plastics may be prepared for accepting 4 copper coatings by chemical etching and surface activation, and the following procedure, illustrate a typical process.

Molded objects of acrylonitrile-butadiene-styrene copolymer were etched by immersion in concentrated sulfuric acid at 50-65 C. for about 15 to 30 minutes. The objects were then rinsed and immersed in a solution prepared by dissolving 2.5 to 5 kg. of stannous chloride in 5 liters of hydrochloric acid of density 1.19, and diluting to liters with water. The temperature of the stannous chloride solution was kept between 15 and 25 C., and the immersion time was about 1 to 3 minutes. The plastic objects were then transferred to a solution containing 0.1 to 0.3 kg. silver nitrate and 1.0 liter ammonium hydroxide, d. 0.9, in 100 liter water at 15 to 25 C. in which they were kept immersed for one to three minutes. They were then ready for copper deposition in any one of the copper deposition baths indicated in the invention hereinabove.

We claim:

1. An aqueous solution for the deposition of copper by chemical reduction containing a source of copper ions, a complexing agent for said ions, a reducing agent for reducing the complex copper ions and as a stabilizing agent 0.001 to 0.5 gram per liter of a compound of the formula wherein X is S or NH; R is hydrogen, lower alkyl, phenyl, carboxy-lower alkyl, or amino; and R is hydrogen, lower alkyl, or nitro.

2. A solution as set forth in claim 1, wherein X is sulfur, R is hydrogen, and R is hydrogen or methyl.

3. A solution as set forth in claim 2, wherein the amount of said stabilizing agent is between 0.001 and 0.05 gram per liter.

4. A solution according to claim 1 in which the stabilizing agent is selected from the group consisting of rhodanine, N-methylrhodanine, N-phenylrhodanine, rhodanine- N-acetic acid, N-aminorhodanine, 3-phenyl-5-nitrorhodanine, S-ethylrhodanine, thiohydantoin.

5. A method of depositing copper on a receptive base from a solution set forth in claim 4 comprising etching such base, rinsing, and immersing said base in a solution as set forth in claim 4.

US. Cl. X.R. 

